1. Field of the Invention
This invention relates to liquid heating systems operated with radiant solar energy and in particular to a control system using a differential amplifier for sensing temperature differentials within the system and controlling the circulation of the liquid responsive thereto.
2. Description of the Prior Art
Solar water heaters have long been known to be an efficient method for utilizing solar radiant energy to heat a liquid such as water for subsequent storage and use. However, the availability of inexpensive electricity and natural gas has heretofore made the solar water heater a second choice due to its high initial capital investment requirement. With the present rapidly increasing costs of conventional sources of power, such as electrical, gas or fossil fuels, the interests in solar water heating systems has been rekindled.
Heretofore the circulation of water within the solar water heating system has been controlled by such methods as a thermostat control, a solar cell control, a clock control or natural convection circulation. A typical thermostat control includes a thermostat mounted on the solar water heater. When the water reaches a predetermined temperature, such as 135.degree. F., the thermostat actuates a control circuit which supplies power to a circulating pump. However, on cold, cloudy or rainy days the solar collector may not heat the water to the predetermined temperature and thus most of the solar energy collected within the solar collector will be lost since the heated water will remain in the solar collector. The typical solar cell control system includes a sun sensor mounted on the roof near the solar collector. When the sun is shining brightly the solar cell actuates circuitry to supply power to the circulating pump. At night or on cloudy days the circulating pump remains off. This solar cell system also wastes some of the solar energy collected on cloudy or overcast days because the solar cell will not be actuated and thus the hot water remains in the solar collector. Furthermore, it is possible for the solar cell sensing system to actually cool the water on days having cold outside temperatures and bright sunshine. This combination of unusual weather conditions is a serious problem in some northern latitudes. The typical clock control system includes a simple electric clock timer which supplies electrical power to the pump at predetermined times, usually turning the circulating pump on after sunrise and off before sunset. The clock control system is simple to design and inexpensive to construct, but on cold or overcast days the water within the solar collector may be cooled rather than heated since the thermal radiation may exceed the solar energy absorption.
A differential temperature sensing and control system avoids all of these previous system deficiencies by measuring the temperature of the water at two locations within the system and controlling the circulation of the water within the system responsive to the temperature differential therebetween. A first temperature sensor is located adjacent to the top. or the hottest section, of the solar collector. A second temperature sensor is mounted on the water storage tank adjacent to the cold water outlet, which is the coldest section thereof. A differential comparator compares the temperature differential between the water storage tank and the solar collector, and circulates the water from the solar collector into the storage tank when the temperature of the water in the solar collector exceeds the temperature of the water in the storage tank by a predetermined level. This differential control system maximizes the solar energy collected within the solar energy collector.
The concept of using a differential comparator for actuating climate control systems is disclosed by Carlson in U.S. Pat. Nos. 3,833,859 and 3,860,837. The Carlson device is merely a thermostat utilizing a differential comparator to sense the temperature differential between the calibration standard and an element which varies in resistance responsive to a change in temperature. Another differential amplifier system for controlling climatic heating and cooling systems is disclosed by Pinckaers in U.S. Pat. No. 3,535,561. The Pinckaers system uses a differential amplifier control system for supplying power to either a cooling system or a heating system responsive to the temperature within a living area.